The Gillard government has come under fire for using $4.3 billion in budget funds to support its carbon pricing policy as companies warned investors of a hit to earnings.

Prime Minister Julia Gillard began the sales campaign for the carbon tax in a relentless series of radio and TV appearances in which she guaranteed not to widen the scope of the price on carbon.

Ms Gillard assured the Seven Network that household petrol costs would stay out of the scheme forever, while noting that the government policy was to remove excise rebates for business such as trucking companies and miners.

Qantas told the ASX that the carbon price would add $110 to $115 million to its costs in the 2013 financial year, that the company could not absorb this and would have to pass all of the amount on to customers in the form of higher prices.

Rival airline Virgin Australia said it faced a cost impact of $45 million in fiscal 2013 and would lift domestic fares by $3, while Regional Express said the tax would decimate air services to the bush. Rex estimates it faces costs of about $6 million a year.

Building materials company Brickworks said the policy would cut earnings by $12.8 million in the same year and that it would have to raises its prices by up to 6 per cent, while Adelaide Brighton said the federal changes would have a $5 million impact on its profit before mitigation.

Treasurer Wayne Swan and Finance Minister Penny Wong faced a series of questions over the budget impact of the policy, which is to widen next year’s deficit by $2.7 billion and cost $4.3 billion over the first four years of the scheme.

The government promised earlier this year that the carbon scheme would be “broadly revenue neutral" and repeated that argument on Sunday when the policy documents revealed the hit to the budget.

Coalition shadow treasurer Joe Hockey said Mr Swan, who he likened to failed businessman Alan Bond, should “come clean" about the impact on the budget.

Mr Swan said on Monday morning the scheme was “broadly budget neutral" over the forward estimates.

But Mr Hockey said $4.3 billion was not a neutral outcome.

“There is no possible way Wayne Swan can seriously think that this plan is ‘broadly budget neutral’," he said.

“This blowout further increases government borrowing, putting upward pressure on interest rates for Australian home buyers and small business."

Mr Hockey told a 300-strong gathering at a hall opposite Mr Swan’s Brisbane electorate office that in the past week, the numbers in the government’s carbon price package had shrunk considerably.

The government initially said the carbon tax would be paid by the 1000 biggest polluters and that had halved.

“Wayne Swan lost 500 companies between midnight and dawn on Thursday," he said.

“It hasn’t taken him long for him to become the Alan Bond of treasurers."

Mr Swan said he had the full support of his party, including former prime minister Kevin Rudd, in selling the federal government’s new carbon tax both domestically and international.

“I’m sure he [Kevin Rudd] is talking to other countries but I think all of our members of parliament, certainly all of our ministers, are talking to communities about putting a price on carbon," he told reporters at the Pacific Palms Home Village in Burpengary north of Brisbane.

“The fact is, when it comes to climate change and pricing carbon pollution, Australia is about the middle of the pack when it comes to the international approach.

“We have to put a price on carbon to make sure we don’t fall behind."

Mr Swan defended the carbon tax economic modelling against claims by the Food and Grocery Council that the impact on prices could be higher than forecast.

The Food and Grocery Council would be given access to Treasury to check the figures, he told ABC Radio in Brisbane.